Referring now to the drawing, wherein like reference numerals refer to like elements throughout, FIG. 1A illustrates the electromagnetic spectrum on a logarithmic scale. The science of spectroscopy studies spectra. In contrast with sciences concerned with other parts of the spectrum, optics particularly involves visible and near-visible light—a very narrow part of the available spectrum which extends in wavelength from about 1 mm to about 1 nm. Near visible light includes colors redder than red (infrared) and colors more violet than violet (ultraviolet). The range extends just far enough to either side of visibility that the light can still be handled by most lenses and mirrors made of the usual materials. The wavelength dependence of optical properties of materials must often be considered.
Absorption-type spectroscopy offers high sensitivity, response times on the order of microseconds, immunity from poisoning, and limited interference from molecular species other than the species under study. Various molecular species can be detected or identified by absorption spectroscopy. Thus, absorption spectroscopy provides a general method of detecting important trace species. In the gas phase, the sensitivity and selectivity of this method is optimized because the species have their absorption strength concentrated in a set of sharp spectral lines. The narrow lines in the spectrum can be used to discriminate against most interfering species.
In many industrial processes, the concentration of trace species in flowing gas streams and liquids must be measured and analyzed with a high degree of speed and accuracy. Such measurement and analysis is required because the concentration of contaminants is often critical to the quality of the end product. Gases such as N2, O2, H2, Ar, and He are used to manufacture integrated circuits, for example, and the presence in those gases of impurities—even at parts per billion (ppb) levels—is damaging and reduces the yield of operational circuits. Therefore, the relatively high sensitivity with which water can be spectroscopically monitored is important to manufacturers of high-purity gases used in the semiconductor industry. Various impurities must be detected in other industrial applications. Further, the presence of impurities, either inherent or deliberately placed, in liquids have become of particular concern of late.
Spectroscopy has obtained parts per million (ppm) level detection for gaseous contaminants in high-purity gases. Detection sensitivities at the ppb level are attainable in some cases. Accordingly, several spectroscopic methods have been applied to such applications as quantitative contamination monitoring in gases, including: absorption measurements in traditional long pathlength cells, photoacoustic spectroscopy, frequency modulation spectroscopy, and intracavity laser absorption spectroscopy. These methods have several features, discussed in U.S. Pat. No. 5,528,040 issued to Lehmann, which make them difficult to use and impractical for industrial applications. They have been largely confined, therefore, to laboratory investigations.
In contrast, continuous wave-cavity ring-down spectroscopy (CW-CRDS) has become an important spectroscopic technique with applications to science, industrial process control, and atmospheric trace gas detection. CW-CRDS has been demonstrated as a technique for the measurement of optical absorption that excels in the low-absorbance regime where conventional methods have inadequate sensitivity. CW-CRDS utilizes the mean lifetime of photons in a high-finesse optical resonator as the absorption-sensitive observable.
Typically, the resonator is formed from a pair of narrow band, ultra-high reflectivity dielectric mirrors, configured appropriately to form a stable optical resonator. A laser pulse is injected into the resonator through a mirror to experience a mean lifetime which depends upon the photon round-trip transit time, the length of the resonator, the absorption cross section and number density of the species, and a factor accounting for intrinsic resonator losses (which arise largely from the frequency-dependent mirror reflectivities when diffraction losses are negligible). The determination of optical absorption is transformed, therefore, from the conventional power-ratio measurement to a measurement of decay time. The ultimate sensitivity of CW-CRDS is determined by the magnitude of the intrinsic resonator losses, which can be minimized with techniques such as superpolishing that permit the fabrication of ultra-low-loss optics.
FIG. 1B illustrates a conventional CW-CRDS apparatus 100 for measuring an impurity in liquid 111 contained within glass cell 109. As shown in FIG. 1B, light 104 is generated from a narrow band, tunable, continuous wave diode laser 102. Laser 102 is temperature and/or current tuned by a temperature and/or current controller (not shown) to put its wavelength on the desired spectral line of the impurity. Focusing lens (or lens system) 106 is positioned in line with light 104 emitted from laser 102. Light 104 exits focusing lens 106 and enters optical cell 112. Optical cell 112 includes mirror 108 at its input side and mirror 110 at its output side. As light 104 enters optical cell 112 it travels along the longitudinal axis of cell 112 and exponentially decays as it repeatedly travels between cell mirrors 108 and 110. The measure of this decay is indicative of the presence or lack thereof of an impurity in liquid 111 contained in glass cell 109.
Detector 114 is coupled between the output of optical cell 112 and processor 116. Processor 116 processes signals from optical detector 114 in order to determine the level of impurity in glass cell 109. A shortcoming of this system is that, although glass cell 109 is oriented at Brewster's angle with respect to light 104, refraction from the surface of glass cell 109 will deviate the path of light 104 away from Brewster's angle as it travels through liquid 111. Adjusting the orientation of glass cell 109 to compensate for the refraction will instead produce external reflection at the exterior surface of glass cell 109. Thus, the walls of glass cell 109, through which the light 104 must travel, inevitably introduce an additional interface which produces undesirable refractions. The result is increased signal loss which decreases sensitivity of the apparatus.
To overcome the shortcomings of conventional systems, an improved system and method for analyzing impurities in liquids using CW-CRDS is provided.